One type of voltage controlled oscillator may conventionally include a ring oscillator, which may have a number of inverting gain stages in a ring. Because the phase noise of the ring oscillator decreases as the output oscillation amplitude increases, a rail-to-rail output signal swing and fast switching (e.g., fast rise and fall times) are desirable in order to improve the noise performance of the oscillator.
From a power consumption point of view, it is desirable to minimize the number of stages in a ring oscillator. If the number of stages in the ring oscillator is decreased by a factor of 2, the oscillator can oscillate at twice the frequency because the total delay around the loop is half as much. Also, the oscillator will dissipate half the amount of power. Therefore it is very appealing to consider ring oscillators with the minimum number of required delay elements.
FIG. 1 is a schematic of a VCO 110 having a single stage ring oscillator of the prior art. The VCO 110 is described in a paper by A. Ahmed, K. Sharaf, H. Haddara, H. G. Ragai, entitled “CMOS VCO-prescaler cell-based design for RF PLL Frequency Synthesizers,” ISCAS 2000, Vol. II, pp. 737-740, May 2000, which is herein incorporated by reference.
For a simplified explanation of the operation of FIG. 1 without any voltage control, let Vcont 154 be high enough so that the resistances in transistors M9 and M10 are low. Also let OUTP 150 be 1 and OUTN 152 be 0 initially. Hence, P transistor M5 130 is on and P transistor M6 132 is off. N transistor M1 114 is on and N transistor M2 112 is off. Node 116 is pulled toward ground, hence P transistor M4 122 is turned on, pulling node 118 to VDD and turning of P transistor M3 120. Also as OUTP 150 is pulled toward ground, P transistor M6 132 is turned on and P transistor M5 130 is turned off. Thus OUTN 152 transitions from 0 to 1. Thus, now OUTP 150 is 0 and OUTN 152 is 1. When OUTN 152 is 1, N transistor M2 112 turns on and OUTN 152 is pulled toward ground. As illustrated the VCO 110 oscillates between VDD and ground.
Another way of looking at FIG. 1 is that the cross-coupled p-channel transistors M3 and M4 provide a first 90 degree phase shift and the cross-coupled p-channel transistors M5 and M6 provide a second 90 degree phase shift. The n-channel transistors M1 and M2 provide the 180 degree phase shift.
To control the frequency of the VCO 110, a single-ended control voltage (Vcont 154) is applied to the gates of transistors M9 124 and M1 0126. Transistors M9 and M10 act as voltage-controlled resistors between nodes OUTP 150 and node 116 and between OUTN 152 and node 118, respectively. As the control voltage Vcont 154 decreases, M9 and M10 will be delayed in switching ON and will have a higher equivalent resistance, resulting in VCO 110 having a lower frequency of oscillation.
The VCO of FIG. 1 has several major disadvantages. First, VCO 110 may not oscillate if the control signal Vcont 154 is too low because transistors M9 and M10 would be an open circuit. Second, the VCO 110 may also fail to oscillate if the signal Vcont 154 is very high and the effective resistance of transistors M9 and M10 is less than the minimum resistance required to sustain oscillation. This lack of VCO oscillation is a potential problem when the VCO is used in a closed loop PLL application, as the phase frequency detector will not have anything to compare against the reference clock frequency.
Therefore, there is need for an improved VCO having a single stage oscillator.